Vitamin E is a major antioxidant, effective in stabilizing unsaturated lipids in cell membranes against autooxidation. Furthermore, vitamin E scavenges free radicals produced by lipid peroxidation and by the normal activity of oxidative enzymes.
Vitamin E is a generic term for tocopherols and tocotrienols, which have saturated and unsaturated phytyl tails, respectively. The α-, β-, γ-, and δ-tocopherols and tocotrienols differ in the number and position of the methyl groups on the chroman ring (FIG. 1 and Table 1). The tocopherols can exist in a number of stereoisomeric forms depending on the chirality of the phytyl tail. Of these compounds RRR-α-tocopherol has the greatest biological activity and accounts for approximately 90% of the vitamin E found in tissues. Natural vitamin E (RRR-α-tocopherol) is a single stereoisomer whereas synthetic vitamin E (all-rac-α-tocopherol) is an equimolar mixture of eight isomers, only one of which is RRR-α-tocopherol. The other seven isomers of synthetic vitamin E have different molecular configurations, all with lower biological activity than RRR-α-tocopherol.